1. Field of the Invention
This invention relates to a structure which can be installed in deep seas and is able to support at its top a plant complex designed for various industrial activities in open sea, in particular it being usable advantageously as a hydrocarbon production platform and a mooring and loading point for oil tankers for sea depths exceeding 1000 m.
2. Description of the Prior Art
Structures such as the braced derrick and articulated derrick have been proposed and designed for hydrocarbon production in deep seas. The braced derrick, being a "yieldable structure" with its first intrinsic period above the wave period range (.gtoreq.30 s) and its second intrinsic period below this (.ltoreq.7 s), has a range of use in terms of water depth which is rather limited, and cannot exceed a bed depth of 500 m.
This structure also appears too complicated, sophisticated and thus costly to allow its use to extend to production in marginal (medium-small) oil and gas fields.
The articulated derrick has the drawback of possessing a critical mechanical member, namely its universal base joint, in a zone which is inaccessible for direct inspection and maintenance. Moreover, the structural discontinuity constituted by said universal joint means that the oil feed conduits which run along said structure must also comprise hinges to allow structural rotation. If the structure is used as a production platform, this configuration does not allow the well heads to be disposed at the surface, but instead requires the use of underwater well heads, leading to a considerable reduction in system reliability and a significant increase in both installation and operating costs.
For the deep-sea mooring of ships, certain of the authors of the present invention have patented (U.S. patent application Ser. No. 393,310 filed on June 29, 1982) a flexible monolithic structure concept with a buoyancy chamber close to its top, having certain apparent analogies with the structure proposed herein.
The structure corresponding to the aforesaid patent has its first intrinsic period above the wave period (.gtoreq.30 s) and its second below the period of waves with a significant energy content (.ltoreq.7 s). This dynamic behaviour limits the application of the concept to a water depth which cannot exceed 500-600 m.
Finally, its method of manufacture and installation, which require it to be constructed, transported and installed in a single monolithic piece, itself constitutes a limit to the depth which can be attained. A further concept which seems to have some analogy with the present invention is the SALM mooring buoy composed of a partially immersed buoy body connected to the sea bed by a vertical chain tensioned by the upward thrust on the buoy. This method cannot be extended to deep seas because, in such a case, in order to ensure the necessary rigidity of the mooring system against horizontal traction, a very high tension (many thousands of tons) would have to be applied to the anchoring line, and this could in no way be withstood by an element of chain type.